The present invention relates to an improvement of an electron tube cathode used for a cathode ray tube for television, or the like, and in particular to an electron tube cathode having an electron emissive material layer containing a rare earth metal oxide, or a heat-resistant oxide as a substitute of the rare-earth metal oxide.
FIG. 9 shows an electron tube cathode used in a cathode ray tube or image pick-up tube for television, disclosed for example in Japanese Patent Kokoku Publication No. S64-5417. In the drawing, reference numeral 111 denotes a base formed mainly of nickel and containing a small amount of silicon (Si), magnesium (Mg) or like reducing element. Reference numeral 112 denotes a cathode sleeve formed of Nichrome.TM. or the like. Reference numeral 115 denotes an electron emissive material layer deposited on the upper surface of the base 111, and containing, as a main constituent, an alkaline-earth metal oxide 121 containing at least barium (Ba), and additionally strontium (Sr) and/or calcium (Ca), and containing a rare earth metal oxide 122 such as scandium oxide of 0.1 to 20 weight percent. Reference numeral 113 denotes a heater disposed in the base 111. The heater 113 heats the electron emissive material layer 115 to emit thermoelectrons.
With the electron tube cathode of the above configuration, the manner of depositing the electron emissive material layer 115 onto the base 111 will next be described. First, a ternary carbonate of barium, strontium, and calcium, and a predetermined amount of scandium oxide are mixed together with a binder and a solvent, to form a suspension. The suspension is sprayed onto the base 111 to a thickness of about 80 .mu.m, and is thereafter heated by the heater 113 during evacuation process of the cathode ray tube. The carbonate of the alkaline-earth metal is converted into alkaline-earth metal oxide. Part of the alkaline-earth metal oxide is reduced and activated to have a semiconducting property, so that the electron emissive material layer 115 consisting of the mixture of the alkaline-earth metal oxide 121 and the rare earth metal oxide 122 is formed on the base 111.
In the activation step, part of the alkaline-earth metal oxide reacts in the following manner. That is, silicon, magnesium and like reducing elements contained in the base 111 move to the interface between the alkaline earth metal oxide 121 and the base 111 by diffusion, and reacts with the alkaline earth metal oxide. For instance, if the alkaline-earth metal oxide is barium oxide, the following reactions (1) and (2) take place: EQU 2BaO+(1/2)Si=Ba+(1/2)Ba.sub.2 SiO.sub.4 (1) EQU BaO+Mg=Ba+MgO (2)
As a result of these reactions, part of the alkaline-earth metal oxide 121 deposited on the base 111 is reduced, to become an oxygen-deficient semiconductor, so that electron emission is facilitated. If no rare earth metal oxide is contained in the electron emissive material layer, operation with a current density of 0.5 to 0.8 A/cm.sup.2, at a cathode temperature of 700 to 800.degree. C. is possible. If a rare earth metal oxide is contained in the electron emissive material layer, operation with a current density of 1.32 to 2.64 A/cm.sup.2 is possible.
Generally, electron emission performance of oxide cathodes depends on the amount of excessive Ba in the oxide. If no rare earth metal oxide is contained, excessive Ba sufficient for a high current operation cannot be supplied, and the current density at which the cathode is operable is small. That is, magnesium oxide (MgO) or barium silicate (Ba.sub.2 SiO.sub.4) which is a by-product generated at the time of the above reaction, and called an intermediate layer is formed, being concentrated on nickel grain interfaces in the base 111 or the interface between the base 111 and the electron emissive material layer 115, so that the rate of the reactions expressed by formulae (1) and (2) above is controlled by the rate of the diffusion of magnesium and silicon in the intermediate layer, and supply of excessive Ba is insufficient.
If a rare earth metal oxide is contained in the electron emissive material layer, the operation is as follows. The following description is made taking scandium oxide (Sc.sub.2 O.sub.3) as an example. During operation of the cathode, at the interface between the base 111 and the electron emissive material layer 115, part of the reducing agent having moved by diffusion through the base 111 reacts with scandium oxide (Sc.sub.2 O.sub.3) in the manner described by the following formula (3), and a small amount of metallic scandium is generated, and part of the metallic scandium forms a solid solution with nickel in the base 111, and a part is retained at the interfaces. EQU (1/2)Sc.sub.2 O.sub.3 +(3/2)Mg=Sc+(3/2)MgO (3)
The metallic scandium generated by the reaction of the formula (3) decomposes the above-mentioned intermediate layer formed on the base 111 or at the nickel grain interfaces in the base 111 in the manner described by the following formula (4), so that supply of excessive Ba is improved, and the rare earth metal oxide in the electron emissive material layer restrains evaporation of excessive Ba, with the result that operation is possible at a higher current density than if no rare earth metal oxide is contained. EQU (1/2)Ba.sub.2 SiO.sub.4 +(4/3)Sc=Ba+(1/2)Si+(2/3)Sc.sub.2 O.sub.3(4)
Japanese Patent Kokai Publication No. S52-91358 discloses a direct-heated cathode having a base formed of a Ni alloy containing a high-melting point metal such as W or Mo which increases the mechanical strength, and a reducing agent such as Mg, Al, Si or Zr, and an alloy layer of Ni-W, or Ni-Mo coated on the surface of the base where an electron emissive material layer is to be deposited.
With the electron tube cathode formed in the described above, the rare earth metal oxide improves the supply of excessive Ba, but the rate of supply of the excessive Ba is controlled by the rate of diffusion of the reducing agent in nickel in the base, and the life characteristic at a high-current density operation of 2 A/cm.sup.2 or more is substantially low.
The latter one of those mentioned above provides an improvement in respect of the thermal deformation which is a problem inherent to the direct-heated cathode emitting thermoelectrons from the electron emissive material layer, utilizing heat generated by the current through the base itself, by coating the base with a layer of an alloy such as Ni-W or Ni-Mo. However, it does not enable operation at a high current density.
With regard to these problems, the assignee of the present application already disclosed in Japanese Patent Application No. H2-56855 (Japanese Patent Kokai Publication No. H3-257735) that it is possible to improve the life characteristics with operation at a high current density of 2 A/cm.sup.2, by diffusion into the base from a metal layer provided between the base and the electron emissive material layer. FIG. 10 shows the configuration of such a cathode.